DESCRIPTION: Ionizing radiation is recognized as a carcinogen in mammals, presumably via the induction of mutations affecting critical autosomal target genes (i.e., oncogenes and tumor suppresssor genes). Though a large body of literature exists documenting ionizing radiation mutagenesis in cultured somatic cells, the relevance of this work to the in vivo situation is unclear. The goal of the proposed research is to obtain basic information about ionizin radiation mutagenesis in somatic cells in vivo. The mutagenic response for bot wild type mice and those with deficiencies for the relevant atm (ataxia telangiectasia) gene will be examined. The autosomal aprt (adenine phosphoribosyltransferase) gene will e used as the mutational target for these studies. This target was chosen because it can detect the complete spectrum of mutational epigenetic events that are known to inactivate tumor suppressor genes. Aprt homozygous deficient clones will be isolated from different tissue of aprt heterozygous deficient mice exposed to 137Cs gamma radiation and a rapid PCR based screening method used to test the hypothesis that deletional events will be induced in the somatic cells of different tissues. This signature mutational event will then be analyzed in detail with molecular and cytogenetic techniques to determine the scale of the deletional events. Other mutational events will also be examined in detail if they are shown to be induced by 137Cs gamma radiation. Next, dose response and split dose experiments will be performed to test the hypothesis that the large deletional events require two independent lesions. Finally, a mouse model for ataxia telangiectasia will be used to test the hypothesis that atm heterozygotes are hypersensitive to ionizing radiation mutagenesis. Completion of the work proposed in this application will provide fundamental information about ionizing radiation mutagenesis in vivo and establish a model system to study the interaction of this important environmental genotoxin with relevant geneti backgrounds.